OP stated: Unless you're already making that much, how does a higher marginal tax rate on income over $500k affect you at all?

So, if you want to make that much some day, then it would affect you when you do.

Further - raising rates on high income earners makes it more likely for others to make more? That seems dubious.

One argument in the original article is that an awful lot of that >$500k income is just non-productive rents levied on the lower income brackets anyway, so discouraging wasteful rentier behavior is a net positive (as it reduces inequality) without reducing productivity at all, since rents by definition aren't productive, they just siphon money into top earners' pockets rather than growing the economy at all.

Decreasing rent-seeking in top earners will leave more money on the table for everyone else.